Background and objective: Seasonal distribution of microbial aetiology in patients with community-acquired pneumonia (CAP) may add important information both for epidemiologists and clinicians. We investigate the seasonal distribution of microbial aetiology in CAP. Methods: This prospective observational study was carried out in the Hospital Clinic of Barcelona, Spain (January 2003–December 2014). Results: We studied 4431 patients with CAP, of whom 2689 (61%) were males. Microbial aetiology was identified in 1756 patients (40%). CAP was most frequent in winter (34%) but two-third of patients with CAP presented in other seasons. Seasonal variations included Streptococcus pneumoniae (winter 21% vs spring 17% vs summer 14% vs autumn 13%, overall P < 0.001). Influenza viruses were most prevalent in autumn (6%) and winter (5%) compared with spring (3%) and summer (1%) (overall P < 0.001). Legionella pneumophila was most frequent in autumn (4%) and summer (4%) compared with spring (2%) and winter (1%) (overall P < 0.001). Incidence of polymicrobial pneumonia also differed between seasons (winter 7% vs spring 5% vs summer 3% vs autumn 6%, overall P = 0.001). We observed a significant correlation between the lowest seasonal average temperature and polymicrobial pneumonia, pneumococcal pneumonia, and influenza viruses; conversely, L. pneumophila was more common when temperatures were higher. Conclusion: CAP should not be regarded as a seasonal disease but occurs throughout all seasons. However, S. pneumoniae, influenza viruses, polymicrobial pneumonia and L. pneumophila are clearly subject to seasonal variations.